Electric compressors used in vehicle air conditioners realize a cooling function by compressing refrigerants with the rotational force transmitted from built-in or connected permanent-magnet type synchronous motors. For the control of motors in such electric compressors, control employing a position sensor, such as a Hall element, cannot be carried out because the interior accommodating the motor is soaked with lubricant; therefore, so-called sensorless control is typically carried out.
In sensorless control of a permanent-magnet type synchronous motor, synchronized operation cannot be carried out immediately because the rotor position at start-up of the electric motor is unknown. Thus, conventionally, a three-phase voltage (current) for rotating the rotor at a constant speed in a particular direction for a particular amount of time is output to the motor to forcibly start it up (for example, refer to Patent Document 1).
Patent Document 1:
Japanese Unexamined Patent Application, Publication No. 2003-28073